Process for introducing gases into liquids in the production of polyphenylene ethers or sulfoxidized parrafins using horizontal reactors

ABSTRACT

A method for introducing gases into liquids, wherein a stirred kettle is used which has the form of a cylindrical vessel in which the length/diameter ratio should be at least 1; and wherein the stirred kettle is filled to a level of more than 70% with the liquid into which gas is to be introduced, the gas feed rate is adapted to the absorption capacity of the liquid, and the stirrer speed is adjusted such that the largest coherent amount of gas is 10% of the volume of the stirred kettle.

This application is a continuation of application Ser. No. 700,027,filed Feb. 11, 1986, now abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a process for introducing gases into liquids,in particular for contacting gaseous and liquid reactants, and to theuse of an apparatus for carrying out this process.

2. Description of the Prior Art

In a number of processes, the problem arises of introducing gases into aliquid, either in order to disperse a gas in a liquid or to dissolve it,or in order to effect a chemical reaction between a gaseous reactant anda liquid reactant.

If a gas, say, is then passed through the liquid, the gas isquantitatively absorbed by the liquid only in the case of a highreaction rate and a small mass transfer resistance. In all other cases,only a more or less small proportion of the gas is absorbed by theliquid. The residual part of the gas remains unutilized.

This is unsatisfactory because the gases used frequently just beprepared at considerable cost.

If, for instance, toxic gases, such as hydrogen cyanide or carbonmonoxide are used, an expensive destruction or reprocessing isindispensable. Difficulties arise also in the cases where the liquidtends to foam under the action of gases.

When air, oxygen or ozone are used for the oxidation of organicsubstances, the formation of highly explosive exit gas mixtures must beexpected. An example of this is the oxidative coupling reaction of2,6-disubstituted phenols. In such cases, a considerable expenditure onsafety engineering is necessary in order to ensure that the reaction canproceed without hazard. Thus, for example, air saturated with toluene isexplosive in the range from about 4° to 42° C.

Numerous types of apparatus are known from the state of the art, whichmake intimate contact of a gas with a liquid possible, for example inUllmanns Encyklopadie der technischen Chemie [Ullmann's Encyclopaedia ofIndustrial Chemistry], 4th edition, volume 1, pages 225 et seq., (inparticular pages 226 and 227), volume 2, pages 275 et seq., and volume3, pages 357 et seq. (in particular page 359). These are plate columnsand packed columns, spray devices, nozzles and stirred kettles which, inaddition to different types of agitator, can contain various furtherinserts. However, the processes working with such types of equipmentfundamentally suffer from the disadvantages described.

European Published specification No. 0,087,670 describes a process foravoiding an explosive gas phase in a vertical gas/liquid reactor with anenveloping tube, closed at the top, and nozzles, through which liquidjets are intended to emerge. This process appears to be involved, sincethe desired gas can be introduced into the reactor only after inert gashas been admitted first and has been so completely dispersed in theliquid that a relatively large coherent gas space is not longer present.

This process has two further disadvantages:

1. The measure of forcing a liquid through nozzles and thus achievingthorough mixing cannot be satisfactorily carried out if the liquid drawnin contains considerable quantities of dissolved gas constituents.

2. As is known, the dispersion of gases by means of jet nozzles requiresa high energy consumption.

There are comparatively few processes which utilise horizontallydisposed reactors.

In a number of scientific investigations (Ando et al., Journal ofChemical Engineering of Japan 5, 193 (1972); Aldo et al., Int. ChemicalEngineering 11, 735 (1971) and Ando et al., AlChE 27, 599 (1981)), theinfluence of individual apparatus characteristics on the gas absorptionand the stirrer power consumption in horizontally disposed reactors wasinvestigated. In these experiments, the fraction of the gaseous phasealways remains markedly above 30% of the reactor volume and the reactionmixture is thus inhomogeneous. This manifests itself in a non-uniformpower consumption by the stirrer.

SUMMARY OF THE INVENTION

It was an object of the present invention to develop a process forintroducing gas into a liquid, in which a separate coherent gas andliquid space in the reactor is avoided. Instead, homogeneous thoroughmixing with the smallest possible power input to the stirrer was to beachieved.

It was an object of a preferred embodiment of the invention to avoid thereprocessing or circulation of toxic or explosive gases or gas mixtures.

It has now been found, surprisingly, that this is achieved when one (ormore) horizontally disposed stirred kettle(s) is or are used and theintroduction of the gas is carried out in the following way:

1. The stirred kettle is filled up to a level of more than 70% with theliquid into which gas is to be introduced.

2. The gas feed rate is adapted to the gas absorption capacity of theliquid.

3. The stirrer speed is adjusted such that the largest coherent gasvolume amounts, as a maximum, to 10% of the volume of the stirredkettle.

Further preferred embodiments are the subject of the dependent claims.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention also comprises the use of the stirred kettles, describedin more detail below, and the application in the oxidative coupling of2,6-dialkylphenols and in the sulphoxidation of paraffins.

The shape of stirred kettle is substantially that of a cylindricalvessel in which the length/diameter ratio is preferably greater than 1.The cross-section of the cylinder is preferably circular. Minordeviations from this preferred embodiment are possible. The size of thekettle is not critical. The stirred kettle should be disposedhorizontally, but slight deviations from this disposition are alsopossible without imparing the inventive effect. Advantageously, thestirrer shaft is disposed axially or at least coaxially. However, slightdeviations are possible even here.

In principle, any known stirrer types can be used, in the kettle (seeUhl, "Gray mixing", volume I, page 19, (1966)). Disc stirrers, barstirrers, grid stirrers, pitched blade stirrers and multi-stage impulsecounter-current (MIG) stirrers as well as Pfaudler-type stirrers haveproved to be suitable in particular. Whereas the conventional gasintroduction reactors have flow-directing inserts, which are intended toimprove the dispersion of the gas in the liquid, the stirred kettle inthe present process preferably manages precisely without such additionalinserts which considerably increase the power input to the stirrer.

Initially, the kettle is filled with the liquid. If no foreign gasesshould be present, the reactor can be evacuated beforehand, if desired.The gas can be fed in either before or after the introduction of theliquid. The gas inlet branch and the liquid feed line can be fitted atany desired point of the reactor.

It can be desirable to introduce the gas continuously and, wherepossible, to create within the stirred kettle a quasi-steady state inwhich the degree of absorption of the gas within the stirred kettleincreases steadily from one side to the other. In such a case, the gasis expediently introduced from that side at which the liquid loaded withgas leaves the kettle. By means of suitable selection of stirrers, aplug-type longitudinal flow in co-current and counter-current flow canbe obtained (compare Journal of Chemical Engineering of Japan 8,472-476, (1975)). It is then possible to withdraw a part stream of theliquid, to which gas has been added only to a defined predeterminedextent. If a gas is employed which reacts with the liquid, with theformation of a reaction product which is not gaseous, the process can becontrolled in such a way that no exit gas at all arises. Nevertheless,it can be advantageous even in this case to stop the conversion after acertain degree of conversion and to remove small quantities of exit gas.There are various possibilities for adapting the gas feed to theabsorption capacity of the liquid, for example by measuring the reactiontemperature or the conversion.

The gas can also be fed from a stock vessel which contains precisely therequired quantity of gas. If it should be necessary for any reasons,either the introduction of gas itself or only the gas feed can beinterrupted at any time.

The largest coherent gas volume should not exceed a maximum of 10%, orbetter 2%, of the volume of the stirred kettle. It is desirable that,even in large kettles, a gas bubble of 10 cm diameter represents thelargest coherent gas volume. The optimum stirrer speed for achievingthis cannot be given in general terms. It depends, inter alia, on thetype of the liquid and the gas, on the absorption capacity of theliquid, on the size and the dimensions of the stirred kettle andespecially on the type of the stirrer. However, an expert is able todetermine the optimum stirrer speed in a few preliminary trials.

The introduction of gas can also be carried out in two or more stirredkettles arranged in series or in parallel.

The introduction of gas is normally carried out under atmosphericpressure. If the solubility of the gas in the liquid is low, it can alsobe advantageous to operate under an elevated pressure. Compared with theprocesses known from the state of the art, the present process has anumber of advantages:

1. It is possible to distribute the entire quantity of gas required inthe liquid over a period of any desired length.

2. In spite of the kettle being filled with only 70% of liquid, it isastonishing that states of flooding, which would cause a collapse of theintroduction of gas, do not arise.

3. Relatively large coherent interfaces between gas and liquid areavoided.

4. It is possible to obtain any desired predetermined conversion in achemical reaction.

5. The stirred kettle manages without flow-directing inserts which wouldlead to an increased power consumption by the stirrer.

6. Due to the uniform distribution of the gas in the liquid, stirringproceeds extremely quietly, with low power input.

7. At a correctly selected stirrer speed, relatively large gas spaces,which could increase the explosion risk, in particular in the vicinityof the hot stirrer shaft, are not formed.

8. Higher space/time yields can be achieved by means of the process.

9. The process is distinguished by simplicity and by high reliabilityand safety from explosions.

The process according to the invention is superior to the knownprocesses according to the state of the art above all whenever a gaseouscomponent dissolves only very slowly in a liquid.

The following may be mentioned here as examples:

1. The oxidative coupling of phenols, disubstituted in theortho-position, to give polyphenylene ethers (compare, for example, B.Buhler, "Spezialplaste [Special Plastics]", Akademieverlag 1978, GermanOffenlegungsschrift No. 3,224,692).

2. The sulphoxidation of paraffins by the light/water process inaccordance with the following simplified reaction equation ##STR1##(compare Ullmanns Encyklopadie der technischen Chemie, [Ullmann'sEncyclopaedia of Industrial Chemistry], 4th edition, volume 22, pages478 et seq., 1982).

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1 and 2 show two embodiments of the reactors according to theinvention.

FIG. 1A shows a stirred kettle, inclined by the angle α from thehorizontal position and having a shaft passing through, in adiagrammatic sectional view 1B. It would be possible, for example, tofeed reaction gas at point 1, to feed reaction liquid at point 4 and towithdraw reaction product at point 2. Item 2 could be used as adegassing branch.

FIG. 2A shows a similar reactor with an eccentric stirrer shaft and adiagramatic sectional view, 2B.

FIG. 3 shows another embodiment of the reactor which is not inclined.

FIG. 3A shows another similar reactor and a diagramatic sectional view,3B.

EXAMPLES Example 1

Discontinuous polymerisation of 2,6-dimethylphenol to polyphenyleneether. Catalyst preparation:

7.8 g of CuCO₃ ·Cu(OH)₂ are dissolved in 23 g of 48% hydrobromic acid,and the homogeneous solution obtained is added to 120 g of morpholine,with stirring.

126 g of the catalyst solution, 4,140 g of toluene, 659 g of methanoland 955 g of a 50% solution of 2,6-dimethylphenol in toluene areinitially introduced into a reactor of 5 litre capacity. 19.7 g ofoxygen are then introduced within 1 hour. The reactor is stirred bymeans of a paddle stirrer at speed of 300 min⁻¹. No gas is removed.Subsequently, the reaction is stopped in the conventional manner byaddition of acetic acid (compare, for example, patent application No. P33 13 864.8).

EXAMPLE 2

Continuous polymerisation of 2,6-dimethylphenol to polyphenylene ether.

1.17 kg/hour of a reaction solution, consisting of 25 g of catalystsolution according to Example 1, 828 g of toluene, 132 g of methanol and191 g of a 50% solution of 2,6-dimethylphenol in toluene as well as 3.94g of oxygen are continuously introduced into a stirred kettle cascadeconsisting of 3 reactors of 5 litre capacity each, according to FIG. 1.Simultaneously, 1.17 kg of reaction solution containing polyphenyleneether are continuously withdrawn from the reactor. The reactors arealways filled with the reaction solution up to a level of more than 70%.The average residence time in the stirred kettle cascade is 45 to 90minutes.

EXAMPLE 3

Sulphoxidation by the light/water process.

Water and fresh paraffin, on the one hand, and SO₂ and O₂, on the otherhand, are introduced continuously into a reactor, as described underFIG. 1, fitted with several high-pressure mercury burners and having atotal volume of 280 litres. The mixture leaving the reactor consists of25 parts of paraffin and one part of an aqueous phase. This mixture isfed to a separator. There is no circulating gas stream. No exit gas isformed.

We claim:
 1. In a method for producing polyphenyl ethers, wherein2,6-dialkylphenols are oxidatively coupled, or a method forsulfoxidation of paraffins by the light/water process, the improvementwhich comprises:carrying out the coupling in at least one horizontallydisposed stirred kettle by way of a method which comprises: adding tosaid stirred kettle up to a level of at least 80-90% of its capacity aliquid into which gas is to be introduced, supplying gas, in the form ofbubbles, to said liquid at a rate adapted to the gas absorption capacityof said liquid, and stirring said liquid, while supplying said gas, suchthat the maximum coherent volume of said gas is 2% of the volume of saidstirred kettle, wherein said liquid and gas form a substantiallyhomogeneous mixture with said gas dispersed in said liquid so thatrelatively large coherent interfaces between gas and liquid are avoidedto thereby reduce chances of an explosion.
 2. A method according toclaim 1, wherein said method is for producing polyphenyl ethers in which2,6-dialkylphenols are oxidatively coupled, wherein said liquid intowhich gas is to be introduced is a solution of a 2,6-dialkylphenol andan organic solvent and said gas is oxygen or gas mixtures containingoxygen, and said solution further contains a copper/amine catalyst.
 3. Amethod according to claim 2, wherein said mixture further contains anactivator.
 4. A method according to claim 1, wherein said method is forsulfoxidation of paraffins by the light/water process, wherein saidliquid into which gas is to be introduced is a mixture of paraffin andwater and said gas is a mixture of sulfur dioxide and oxygen.
 5. Themethod of claim 1, wherein said stirring is carried out by a discstirrer, a bar stirrer, a grid stirrer, a pitched blade stirrer, amulti-stage impulse counter-current (MIG) stirrer or a Pfaudler-typestirrer.
 6. The method according to claim 1, wherein a gas bubble havinga diameter of 10 cm represents the largest coherent volume of said gas.7. The method according to claim 1, wherein introduction of gas iscarried out continuously.
 8. The method of claim 1, wherein saidhorizontally disposed stirred kettle is a cylindrical vessel having aratio of length to diameter of at least 1 and which includes an axial orcoaxial stirring shaft.